1. Field of the Invention
The present invention relates to a surface-mount type coil component having a mount surface for mounting the same on a printed circuit board or a hybrid IC (HIC).
2. Description of the Related Art
Known coil components mounted on circuits in electronic apparatus such as personal computers and cellular phones include wire-wound type components provided by winding a copper wire around a ferrite core, multi-layer type components provided by forming a coil conductor pattern on a surface of a magnetic sheet made of ferrite and stacking such magnetic sheets one over another, and thin-film type components provided by forming insulation films and coil conductors constituted by metal thin films alternately using thin film forming techniques. Multi-layer type and thin-film type coil components can be easily provided with a small size.
Known coil components include common mode filters for suppressing a common mode current which can cause electromagnetic interference in a balanced transmission system. Patent Document 1 discloses a common mode choke coil (common mode filter) provided by stacking a first insulator layer, two lead-out electrodes, a second insulator layer, a first coil conductor, a third insulator layer, a second coil conductor, and a fourth insulator layer in the order listed on a surface of a magnetic substrate.
One of the two lead-out electrodes is put in contact and electrical connection with an inner circumferential end of the first coil conductor through a via hole provided on the second insulator layer, and the other lead-out electrode is put in contact and electrical connection with an inner circumferential end of the second coil conductor through via holes provided on the second and third insulator layers. The regions for connecting the coil conductors and the lead-out electrodes are formed inside the coil conductors when viewed in the normal direction of a substrate surface of the magnetic substrate. Therefore, the via holes are formed inside the coil conductors when viewed in the normal direction of the substrate surface.
In many common mode filters, insulator layers inside two coil conductors are removed to form an opening, and a magnetic layer is formed so as to fill the opening in order to improve the degree of mutual magnetic coupling between the two coil conductors and to improve impedance characteristics by increasing common mode impedance.
Recently, there is demand for reduction in the size of common mode filters for use in small electronic apparatus such as cellular phones. Reduction in the size of common mode filters results in a need for reducing areas occupied by via holes. Via holes are frequently formed using a photolithographic process to form the holes with a very small size.
However, via holes are formed greater than the width of coil conductors in most cases for the purpose of improving the reliability of connection between coil conductors and lead-out electrodes and because of photolithographic conditions such as the resolution of photosensitive resin used as insulator layers in which the via holes are formed. In the case of a common mode filter disclosed in Patent Document 1, connecting regions for inter-layer connection between coil conductors and lead-out electrodes must be provided inside the coil conductors, and a part of regions for forming coil conductors and magnetic layers must be spared for the provision of via hole regions. Thus, connecting regions must be provided separately from the region where coil conductors and magnetic layers are formed. As a result, limitations are placed on attempts to increase the number of turns of a coil conductor or the surface area of a magnetic layer, and it is therefore difficult to obtain a common mode filter having high impedance. A problem therefore arises in that it is difficult to obtain a common mode filter which is excellent in electric characteristics such as impedance characteristics.
Patent Document 2 discloses a thin-film common mode filter provided by stacking a first insulator layer, a lower lead conductor, a second insulator layer, a lower coil conductor, a third insulator layer, an upper coil conductor, a fourth insulator layer, an upper lead conductor, and a fifth insulator layer in the order listed, on an insulated magnetic substrate. One end of the lower lead conductor is electrically connected to an end of the lower coil conductor at the center thereof through a through hole formed in the second insulator layer. One end of the upper lead conductor is electrically connected to an end of the upper coil conductor at the center thereof through a through hole formed in the fourth insulator layer. An insulated magnetic element is formed inside the two coil conductors to provide a return part in a magnetic path in the middle of the coils.
As apparent from above, it is also required in the thin-film common mode filter disclosed in Patent Document 2 to connect coil conductors and lead conductors at the centers of the coil conductors. Therefore, the above-described problem is also encountered in the thin-film common mode filter disclosed in Patent Document 2. The above-described problem occurs not only in common mode filters but also in other multi-layer type and thin-film type coil components having spiral coil conductors such as inductors and transformers.
Patent Document 1: Japanese Patent No. 3601619
Patent Document 2: JP-A-2005-159223